This invention relates to fin strip material, particularly, although not exclusively, as used in heat exchange tubes as a secondary heat transfer material. Heat transfer capabilities tend to be affected by the amount of secondary surface area made available in a given flow passage. However, fin density cannot be increased without regard to the extent to which the pressure drop of a flowing fluid may be affected. Also, and especially in dealing with a fin strip which must be bent to an annular configuration for installing between concentric tubes, adjacent fins crowd upon one another along the inner periphery of a curved strip and consequently reduce open flow area. In light of these considerations, it is the practice in the prior art to select a fin strip which, in terms of fins per inch, is essentially a compromise between what the designer would like to achieve in heat transfer performance and what is possible to achieve while maintaining an acceptable volume of flow over the fin surface. It has been suggested that the problem of crowding along the inner periphery of a bent strip can be overcome by forming the fin convolution so that valley portions are more widely spaced apart than are peak portions. See DeGroote U.S. Pat. No. 3,831,247, dated Aug. 27, 1974. This lessens flow and pressure drop problems but does so at a cost in overall fin density. Neither does it provide substantially uniform fin density with substantially uniform open flow area between fin surfaces. Insofar as is known, the problem of achieving high and uniform fin density without seriously affecting volume flow and pressure drop has not heretofore been successfully dealt with.